Much improvement has been made in recent years in the non-surgical therapy of cancer. However, permanent tumor regression induced by anti-tumor chemotherapy, radiotherapy, immunotherapy, or combinations of these treatment modes, is confined to a few types of malignancies. Part of the difficulty in the application of non-surgical anti-tumor therapy is based on relative ignorance of the cell kinetics that underlie tumor regression. According to current clinical experience, it seems likely that combinations of chemotherapy, radiotherapy, and immunotherapy offer the best hope for effective non-surgical treatment. An understanding of the cell kinetic mechanisms of tumor regression should form a rational framework for the integration of these non-surgical modes of cancer therapy. Thus, we propose to study the cell kinetics that underlie tumor regression. The specific roles of cell killing, inhibition of DNA synthesis, reduction of cell proliferation, and alteration of the time parameters of tumor cell replication will be determined by a combination of quantitative autoradiography and DNA cytofluorometry. A transplantable spindle cell tumor induced in mice by UV irradiation will be employed as a model. This tumor will not grow in immune competent syngeneic hosts, but grows progressively to lethal size in thymectomized, X-irradiated hosts. In hosts receiving X-irradiation alone, the tumor grows temporarily, but then undergoes complete regression. Determination of the cell kinetic parameters that characterize progressive growth and regression will specify the kinetic mechanisms of these two patterns of tumor growth in this model tumor system.